The present invention is generally directed to rotating disk electrostatic precipitators for removing particulate matter contained in a gas stream. More particularly, the present invention is related to a particular configuration and design for a rotating disk electrostatic precipitator which includes a removable duct which provides a uniform flow path for the gas between the rotating precipitator disks or plates.
There are many industries which generate flue gases or other gases which are laden with particulate matter. These industries include, for example, smelting, steel manufacture and electric power generation, particularly the geneeration of electric power from coal combustion. Many methods have been employed to remove this particulate matter from flue gases to prevent its being deposited in the atmosphere. One of the methods employed is that of electrostatic precipitation in which the dust laden gas is passed between high voltage electrodes. In general, such systems operate by imparting an electrical charge on the individual particles in the gas stream. The charged particles are then made to migrate to a collection surface by the action of an applied potential difference. While the aforementioned principles of operation are fundamentally simple, there are many secondary factors which tend to complicate the process. For example, particle size and particle size distribution tend to play an important role in collection efficiency since particle mobility is largely dependent upon particle size. Similarly the travel distance between the point at which the particle is charged and the collecting surface is also important as is the gas flow velocity. Furthermore, the chemical and electrical nature of the dust itself is particularly important. For example, high resistivity dusts deposited on the collecting plates tends to increase the possibility of back corona discharge as a result of poor charge leakage to the collecting plate. Additionally, such dusts also create problems of flash over and reentrainment of the particulate matter in the gas stream. All of these phenomena negatively impact collection efficiency. In particular, the term collection efficiency refers to the amount of electrical energy which must be supplied to a precipitator system in order to remove a specified mass of particulate matter from the gas in which it is entrained.
At present, electrostatic precipitators of the kind contemplated herein are large structures. In a typical present day precipitator, dust is collected on a plurality of large vertical plates which are up to about 20 feet high and 20 feet long. These plates are rapped occasionally to cause the collected dust to drop off in clumps and fall to the bottom of the precipitator from which it is removed. The average dust layer on such plates may reach 1/8 or even 1/4 inch in thickness. As discussed above the presence of this dust layer is known to cause problems of reentrainment, back corona and flashover. However, experiments by the instant inventor have indicated that not only are these problems absent when the collection surfaces are kept clean, but, unexpectedly, the collection efficiency can be made as much as 10 to 20 times as high as it is when the surfaces are coated with even a very thin layer of dust. The reason for this phenomenon is not yet fully appreciated but it appears to be related to the greatly improved charge transfer that occurs when dust is collected on bare conducting surfaces. Accordingly, methods of continuously cleaning the collection surfaces is greatly desirable. More particularly, the present invention employs a rotating disk electrostatic precipitator design incorporating wiper means for continuously removing accumulated dust from the collecting disks. Even more particularly, the present invention provides an electrostatic precipitator configuration which includes a readily removable duct which operates to provide a uniform and controlled flow of gas through the precipitator itself.
Accordingly, the precipitator of the present design avoids the problems encountered in other designs employing rotating electrostatic precipitator disks. For example, U.S. Pat. No. 3,929,436 issued to Kim et al. on Dec. 30, 1975 discloses a rotating electrostatic precipitator design in which the particulate laden gas is seen to traverse an arcuate path. Such flow paths are unnecessarily inefficient in that flow velocity imparted to the gas in traversing the precipitator tends to impart a radially outward centrifugal force to the entrained particles thereby resulting in uneven particulate distribution, collection and deposition of particles on the outer walls of the precipitator. Similar centrifugal velocitites problems are also present in the precipitator design described in U.S. Pat. No. 2,631,687 issued Mar. 17, 1953 to F. J. Dohrer. However, the design of the precipitator of the present invention avoids these difficulties and at the same time provides a precipitator assembly which is capable of easy dissassembly for cleaning, servicing and replacement of wiper blades. Moreover, the precipitator design of the present invention may be easily ganged in a variety of series and parallel arrangements for optimal efficiency and particulate matter removal.